For more than two decades now, the use of a 002 sensor as a standalone fire detector or in combination with smoke detectors has been continually advocated by experts as the most effective fire detector. The reason is two-fold. First, there is a significant advantage of using a 002 sensor rather than a smoke detector for fire initiation detection. The mobility of 002 as a gas is far greater than that for smoke which is much heavier. Therefore CO2 diffuses from the fire to the detector in a much shorter time leading to a detector with a faster response time for enunciating a fire. Second, over the past two decades, compact, low cost and reliable NDIR type CO2 sensors have become readily available. As a matter of fact, over the same period of time, a large number of deployment schemes, fire fighting techniques and fire control strategies, which use either a standalone NDIR 002 sensor or in combination with smoke detectors, have been advanced. The most notable proposals of such are summarized as follows.
In U.S. Pat. No. 5,053,754 (1991), Wong advanced the first NDIR 002 sensor used as a standalone fire detector. A fire detection system using at least two NDIR 002 sensors positioned at spaced locations in an area for pin-pointing the exact origin of a fire was described in U.S. Pat. No. 5,079,422 (1992) by Wong. Meanwhile a standalone and compact low cost fire detector which responds quickly to an increase in the concentration of 002 gas in the ambient air was advanced in U.S. Pat. No. 5,103,096 (1992) by Wong. In U.S. Pat. No. 5,369,397 (1994), an adaptive fire detector taking advantage of the capability of an NDIR CO2 sensor for computing the rate of 002 increase to shorten the response time for enunciating the onset of a fire was also advanced by Wong. In U.S. Pat. No. 5,592,147 (1997), an NDIR 002 sensor used cooperatively in combination with a photoelectric smoke detector for significantly reducing false alarms was put forth by Wong. Also in 1997 and in U.S. Pat. No. 5,691,704, Wong disclosed another NDIR CO2 photoelectric smoke detector combination fire detector with special software which can be designed into a single semiconductor chip for cost reduction and further false alarm reduction improvement. In U.S. Pat. No. 5,767,776 (1998), Wong disclosed the design of an NDIR 002 and smoke detector combination fire detector which reduces the maximum average fire enunciation time to less than 1.5 minutes. Further refinement of this design was described in U.S. Pat. No. 5,798,700 (1998) by Wong, U.S. Pat. No. 5,945,924 (1999) by Marman et al. and U.S. Pat. No. 5,966,077 (1999) by Wong. Finally, a method for dynamically adjusting the criteria for detecting fire through smoke concentration using an NDIR 002 and smoke detector combination was described by Wong in U.S. Pat. No. 6,107,925 (2000).
From the methodology listing and discussion elucidated above involving an NDIR 002 sensor used either as a standalone fire detector or in combination with a smoke detector, the advantages in fire detection, both in the resistance against frequent false alarms and the provision of faster response to flaming or fast-moving fires, are quite obvious and cannot be easily denied. Yet today such a fire detection scheme has yet to be taken advantage of. There are several reasons for this. Even with the drastic cost reduction for present day NDIR CO2 sensors, the cost is still too high when compared with ionization type smoke detectors. In addition, when an NDIR gas sensor operates continuously it tends to consume quite a bit more power than conventional smoke detectors, thereby posing an operational burden.
The most common smoke detectors currently in use today belong to two types. The first type is the so-called ionization smoke detector best for detecting almost invisible smoke particles ranging in size from <1.0 microns to −5 microns. The second type is called the photoelectric smoke detector best for detecting visible smoke particles >5 microns in size. In recent years, photoelectric smoke detectors, because of their higher cost (−$30 retail), have fallen significantly behind ionization smoke detectors in sales. Combined ionization and photoelectric smoke detectors, albeit at an even higher cost (−$40 retail), have also been available for quite some time but have not to date received much acceptance by the general public.
Despite their low cost, relatively maintenance-free operation and wide acceptance by the general public, the smoke detectors in widespread use today are not without problems and certainly far from being ideal. One of the biggest problems with smoke detectors is their frequent false-alarm. By the nature of their operational principle, any micron-size particulate matter other than smoke from an actual fire can potentially set off the alarm. Kitchen grease particles generated by a hot stove is one classic example. Over-zealous dusting of objects and/or furniture near the detector is another. Frequent false-alarms are not just harmless nuisances, they can potentially tie up limited fire-fighting resources in many locales to fight real fires. Some people actually disable their smoke detectors by temporarily removing the battery in order to escape such annoying episodes. This latter situation could be outright dangerous especially when these people forget to rearm their smoke detectors afterwards by putting back the battery.
The present invention seeks to change the nature of smoke detectors used today. It proposes a smoke detector that does not suffer the drawbacks of an ionization smoke detector or a photoelectric smoke detector and yet it also does not suffer the drawbacks associated with prior attempts to either use an NDIR 002 sensor as a standalone fire detector or in combination with smoke detectors.